Low pressure fluorescent and discharge lamps



G. MEISTER ETAL' 2,802,129 Low PRESSURE FLUORESCENT AND DISCHARGEv LAMPSFiled Aug. 21, 1952 Aug. 6, 1957 Mnited States Patent C LOW PRESSUREFLUORESCENT AND DISCHARGE LAMPS George Meister, Newark, and Thomas H.Heine, Cedar Grove, N. J., assignors to Westinghouse ElectricCorporation, East Pittsburgh, Pa., a corporation of Penn- SylvaniaApplication August 21, 1952, Serial No. 305,664

2 Claims. (Cl. 313-109) This invention relates to low pressure dischargelamps and, more particularly, to such employing only mixtures of raregases and mercury as the filling material.

The principal object `of ourv invention, generally considered, is toproduce low pressure iluorescent and discharge lamps having a lling of amixture of mercury and rare gases of such a composition that uorescenceis etiiciently excited in the phosphor.

Another object of our invention is to produce a low pressure fluorescentor discharge lamp comprising a mixture of rare gases in which xenon ismixed with neon and mercury in the proportion to get good efficiency atpractical pressures.

A further object of our invention is to produce a fluorescent ordischarge lamp which has characteristics such as above outlined, wherebyit is better than a lamp having only one rare gas admixed with mercuryvapor because it has a high ultra-violet and corresponding luminousetliciency.

Other objects and advantages of the invention will become apparent asthe description proceeds.

Referring to the drawing:

Figure l is an elevational view, with parts in longitudinal section of alamp embodying our invention.

Figure 2vis a graph showing the variation in operating voltage as thecomposition of the contained gas is changed, each curve having appliedthereto a numeral indicating pressure in millimeters of the enclosedgas.

Figure 3 is `a graph `showing the variation in arc or visible lightoutput eiiiciency as the composition of the contained gas is changed,each curve having applied thereto a numeral indicating pressure inmillimeters of the enclosed gas.

Figure 4 is a graph showing the variations in ultraviolet outputetticiency as the composition of the contained gas is changed, eachcurve having applied thereto a numeral indicating pressure inmillimeters of the enclosed gas.

Figure 5 is a graph showing the variations in uorescent light outputetliciency as the composition of the contained gas is changed, eachcurve having applied thereto a numeral indicating pressure inmillimeters of the enclosed gas.

As is well known, mercury vapor admixed with an inert or rare gas, suchas argon at low pressure, is commercially employed for the generation ofultra-violet radiations which may excite phosphors to give olf visibleradiations in uorescent discharge lamps.

The electrical characteristics of such vlow pressure mercury dischargesare known. It has been shown that, at a given lamp temperature, pressureand current, as the atomic weight of the inert gas increases, thevoltage gradient in the mercury discharge and the voltage drop at theelectrodes decrease. l

The early data was mainly taken on the individual inert gases mixed withmercury. We have investigated the use of mixtures of inert gases withmercury for discharge lamps, as a continuation-in-part of the subjectmatter described and claimed in ourapplications, Serial No. 102,016,tiled June 29, 1949, now Patent No. 2,714,684;

Serial No. 243,612, filed Aug. 25, 1951, now Patent No. f

2,714,685; and Serial No. 296,042, led lune 27, 1952, now Patent No.2,714,682; and owned by the assignee of the present application.

ln order to obtain as much information as possible on one experimentallamp, a special tube was designed in which the central portion consistedof a 5 inch section of glass identified by the Corning trademark Vycor(as further identied in the Rentschler Patent No. 2,469,410, dated May10, 1949) and of 1.5 inches inside [diameter which Awas sealed to a 1.5inch diameter glass tube, identified by the Corning trademark Pyrex, ateach end. The Pyrex sections could be coated with phosphors.Independently heated oxide coated cathodes were used and spaced about 24inches apart. In addition, a water-jacket three inches in diameter wasdesigned to fit around the lamp in which the central portion again was a5 inch Vycor section sealed with white wax to a Pyrex section at eachend. With this construction it was possible to measure the light outputof the arc discharge, and the light output of the arc, at different gaspressures and controlled temperatures with corresponding voltages andpower input.

The assembled tube with coated yphosphor sections was sealed onto anexhaust system which had a cooling trap, surrounded by Dry Icethroughout the experiment. A liter reservoir for allowing properdiffusion of gas mixtures was provided into which could be introducedthe spectroscopically pure inert or noble gases which were employed. Thesystem also was equipped with a Mcf Leod gauge to read pressures.

The lamp was exhausted and baked at 475 C. for about one hour. Thecathcdes were processed until no more gas was liberated. Thewater-'jacket was'put into place so that the Vycor sections of thejacket and lamp Vycor sections coincided and was iirmly fastened to keepit in a fixed position. The annular space between the lamp and jacketcould be flushed with water at any desired temperature, which was readon a thermometer placed in the water surrounding the lamp. A voltagestabilizer was used to control the voltage input on the lamenttransformers, the discharge transformer, and the ultra-violet meter.

The ultra-violet meter containing a tantalum cell to read the 2537 A.radiation was set in position opposite the Vycor section. Thephoto-valtaic cells were placed in position opposite the Vycorsectionand the phosphor sections. They were checked before and after aset of readings with a standard incandescent lamp, also using a voltagestabilizer in the circuit. All stands were securely fastened so as tokeep all positions iiixed during a run.

The lamp was seasoned for several days before any readings were taken.This seasoning was done in mercury vapor alone, continuously exhaustingduring the entire period. During operation of the lamp, water wasflushed up and down the tube until the Water temperature was 45 C. Allthe data were subsequently taken at this temperature. The temperaturewas checked before and after each current reading. As each currentsetting was made, the ultraviolet output of the arc, the voltage, thevisible light output of the arc, and the light output of the phosphorwere read in that order. These zero gas pressure data were againobtained as a reference check after each change of inert gas or inertgas mixture. After all the preliminary data were obtained, then thislamp was operated at diierent inert gas pressures and mixtures of inertgases. The inert gases investigated were xenon, neon and mixtures ofxenon and neon. These gas mixtures were varied so as to obtainsuflicient data to show any difference in characteristics and each onewas dilused at least 16 hours to insure uniform mixing.

The following mixtures were used:

Neon, percent Xenon, percent reference point.

VOLTAGE CHARACTERISTICS Curves plotted to compare the voltage with thecorresponding composition atconstant current and temperature forxenon-neon mixtures show that the lowest voltage is obtained with 2 mm.gas pressure throughout the entire range of composition from about 8%xenon- 92% neon to 100% xenon. Outside of this range, the lowest voltageis obtained with l mm. gas pressure. These curves, therefore, generallyagree with previous data obtained with the krypton-argon andKrypton-neon mixtures. However, in the neon-rich Krypton-neon mixtures,the lowest voltage also was obtained with 1 mm. gas pressure.

Figure 2 presents the relative voltage of xenon-neon mixtures and showsa rapid drop in voltage as the xenon concentration `is increased. Therate of decrease in voltage diminishes at high xenon proportions until`above about 75% xenon there is comparatively little change, especiallyat 2, 3 and 4 mm.

The l mm. curve is interesting in that it crosses all other curves andshows a higher voltage than for any other in the range of about 70 to92% xenon. It is possible that in this range at 1 mm. the mean free pathis of such a length that ionizing collisions are less frequent than athigher pressures, thereby resulting in a higher voltage. When thepressure is raised to 2 mm., the optimum mean free path is approached sothat there is the proper acceleration of electrons between collisionsfor maximum ionization. Then at higher pressures the mean free path isshortened to such an extent that ionization can only be meantained by anincrease in the voltage gradient.

The voltage for pure xenon is practically independent of the gaspressure for l, 3, 4 and 5 mm. and amounts to approximately 95% of thevoltage of pure krypton at 2 mm., while for 2 mm. gas pressure it isapproximately 90.5%.

A statement was made that the voltage in these low pressure mercurydischarges containing inert gases varies directly with the ionizationpotential of the gas. This relation is shown by plotting the voltageagainst the atomic number ot' the gas which is correlated to theionization potential. This correlation is further shown when therelative percentage voltage values and ionization potential values aretabulated with krypton as the reference point (Table l), at 2 mm.

Table I Atomic Number Ionization Percent Potential Percent Voltage 4 ARCCHARACTERISTICS Figure 3 shows the arc eciency curves for mixtures ofxenon and neon and the characteristic to have a very pronounced maximumat 1 mm. This occurs because as the gas pressure producing maximumefficiency goes from about 2 mm. for pure neon to 0.2 mm. for pure xenonit passes through the 1 mm. range near the composition of 40% xenon-60%neon. The 2 mm. curve shows relatively little change until more than 50%xenon is added, after which the efficiency drops rapidly. At higherpressures the addition of only small Iamounts of xenon has a deliniteeffect in reducing eciency.

It is interesting to note that the arc visible output eiliciencies forpure xenon show approximately the same large decrease as later observedin the ultraviolet and Fluorescent-composition curves, shownrespectively in Figures 4 and 5. On the other hand, pure neon at allpressures from 1 to 5 mm. maintains a high efficiency showing a drop ofonly about 9% over that pressure range. These curves, for pressures downonly to l mm. compare with the curves previously published on thekryptonfneon series in that they also showed the highest eiciency at 1mm. pressure over approximately the same composition range. Forneon-rich mixtures greater than neon, the eciencies at 1 mm. pressuredecrease rapidly so that at pure neon the efficiencies for 1 and 4 mm.pressures are approximately alike (Fig. 3). This rapid decrease inefficiency was also observed with krypton-neon mixtures when the neoncontent exceeded 80%. However, at pressures below 1 mm., the etciency ofa 75% xenon-25% neon and 50% xenon-50% neon mixture reaches a maximumwhich is higher than the corresponding values for other pressures. Thefollowing table presents the maximum visible output efficiencies and thepressures at which they occur, for representative mixtures with kryptonat 2 mm. taken as the 100% reference point (Table 2).

Composition Pressure, Eciency, mm. of Hg Percent Xe Ne It is noted thatthe maximum occurs at 0.2 mm. gas pressure for pure xenon and thenshifts gradually up to 2.3 mm. gas pressure for pure neon. This issimilar to the characteristics previously published on krypton-neonmixtures which showed that the gas pressure for maximum eciencygradually rose from 1 mm. for pure krypton up to approximately 2 mm. forpure neon. In the case of krypton-argon there was no shift in gaspressure for maximum e'iciency. In connection with these maximum arceciencies, it should be noted that they do not occur at the pressure forminimum voltage which is the case with ultra-violet and uorescent outputefficiencies.

ULTRA-VIOLET CHARACTERISTICS In the case of xenon-neon, Figure 4, amaximum in efficiency occurs for a mixture of 75% xenon, 25% neon, at 1mm. The highest eiciency of mixtures at 2 mm. also occurs for thiscomposition. At 3 mm. there is very little change in etl'iciencythroughout the entire range of compositions. At higher pressures theaddition of xenon causes a decrease in eciency.

FLUORESCENT CHARACTERISTICS In general the fluorescent output efficiencyvs. composition curves at constant current and temperature for xenon-`neon mixtures (Fig. 5) have the same general shape and relative outputas the ultraviolet eiciency v. composition curves at the correspondinggas pressures.

The highest fluorescent output efliciency is obtained at approximately 1mm. gas pressure for pure xenon and for mixtures down to about 42%xenon-58% neon. Below this composition the highest iiuorescenteliiciency occurs at 2 4mm. gas pressure, and the eiciency thengenerally progressively decreases as the pressure increases. Theefliciency of the l mm. curve continues to decrease as the gascomposition becomes richer in neon until at pure neon the etlciency at 1mm. equals that at 5 mm. gas pressure.

The fluorescent output efliciency for pure neon is better than that forpure xenon at all pressures of 4 mm. and above. As the pressureincreases from 4 to 5 mm., the fluorescent output eliiciency for themixtures generally exceeds that of pure xenon, so that at 5 mm. thelluorescent eiciency for xenon is only about 66% that of xenon at l mm.The relative decrease in fluorescent Output eliiciency is approximatelythe same as with the ultraviolet eiciency.

So far, the matter of voltage, arc eiciency, ultra violet efficiency orfluorescent output efliciency only, Ihave been discussed. We now come tothe bearing that voltage has on fluorescent light output. Aconsideration of the voltage curves will show that they increase towardthe pure neon end of the range, thereby effecting a correspondingincrease in total output as the percentage of neon is increased, eventhough the eiciency is not at a maximum, especially at the lowerpressures. This effect is illustrated in the following:

80% xenon- 20% argon' mixture. '."Ihese neon-rich mixtures, therefore,would not only be less expensive and give more total light output, butsuch would be obtained at only a slight sacrifice in the luminousefficiency obtainable from these xenon-neon mixtures.

' SUMMARY The use of xenon as the inert gas filling in a low pressuremercury discharge lamp has been restricted in the past by severaldisadvantages, such as the high cost of pure Xenon, instability and-striations in the discharge at reduced temperatures, and low powerconsumption, with a resulting low total light output compared tostandard fluorescent lamps. The addition of neon to xenon in a lowpressure mercury discharge lamp, to les-sen previously citeddisadvantages, also increases the ultra-violet output and thus thephosphor output etiiciency of the discharge, over that of neon or xenonwhen used alone.

While the ultra-violet and uorescent characteristics of the neon-xenonmixtures are somewhat similar at both l and 2 mm. gas pressures, whichis the desirable range of pressure for practical lamps, a discussion ofeach has been made. All relative percentage values are based on thecharacteristics of 100% krypton and 2 mm. gas pressure.

Thus by the use of 75% xenon-25% neon instead of 100% xenon at l to 2mm. gas pressure, it can be seen that a higher output lamp operating ata higher eiiciency will result, which at the same time will be lesscostly, since neon is much cheaper than xenon and the discharge wouldoperate satisfactorily at a lower temperature because neon is known toenhance low temperature pertures, from Figs. 2 and 5 Pressure: 1 mm.Pressure: 2 mm.

Xe, Ne, percent percent Eti- Etnclency V VA Total ciency V VA TotalOutput Output Pressure: 3 mm. Pressure: 4 mm.

From the foregomg table, 1t will be seen that since the formance. It 1sfurther to be noted that this gas nuxluminous iiuorescent eiiiciency,with 2 mm. total gas pressure for a 40% xenon-60% neon mixture, ispractically the same as pure xenon, the less expensive neon can besubstituted up to and thereby obtain a total light output which is about25% higher than that from pure xenon, and about 19% higher than theapproximately 75% xenon-25% neon mixture giving maximum luminouseiciency.

Similarly, when the total gas pressure is 1 mm., a 40% xenon- 60% neonmixture would have a total light output which is about 27% higher thanthat when using pure xenon, and about 19% higher than the light outputof an 80% xenon-20% neon mixture where the highest eiciency isapproached. A` 20% :renom-80% neon mixture at 2 mm. pressure will givean additional 17% total light output over that of the 40% xenon-60% neonmixture, and at l mm. gas pressure the total light output would be about30% higher than that for the ture is not intended only for iluorescentlamps, but also for ultraviolet lamps. Any phosphor that is excitedmainly by short u. v. (2537 A.) is suitable for these 11u0- rescentlamps. That is why there is such good correlation between u. V. andiluorescent efficiency, as illustrated by the curves.

Fluorescent lamps, one of which is illustrated in lFigure l, exhibi-tthese experimental data. The lamp of said ligure comprises an elongatedtranslucent vitreous envelope 11, with heated tilamentary electrodes l2and 13, one in each end portion, and containing the selected noble gasmixture and some mercury, indicated by the globule 14. If a fluorescentlamp, the selected phosphor 15 is applied to the inner surface of theenvelope.

Although the experimental lamp described contained a 3500" whitephosphor of zinc beryllium silicate and magnesium tungstate, thephosphor 'for commercial use may be any one which eiciently uses and,therefore, has a "7 high absorption of, ultraeviolet radiations, in theregion of 25.357 A. U., that is, the mercury resonanceradiati'on, andconsequently a strong ultra-violet response giving.- a.- good lightoutput. Another example of phosphors which `may be employed are the.halo phosphates described in the British Patent No. 578,192.

Although preferred embodiments have been disclosed, it will beunder-stood that modilications may be made within the spirit and scopeof the invention.

We claim.:

1. A lowpressure positive-column mercury-discharge lamp comprising anelongated and light-transmitting envelope, an electrode in each endportion of said envelope, a small charge of mercury contained withinsaid envelope, a contained mixture Within. said envelope of Xenon and'neon gas at a pressure of from l to. 2 mm, of mercury, the proportion ofxenon being from 60% to 90% and the proportion of neon being from 40% to10%.

2. A low-pressure positive-column mercury-discharge References Cited inthe le of this patent UNITEDy STATES` PATENTS 1,726,107 Hertz Aug. 27,1929 1,858,698 Zons May 17, 193,2 2,346,522 Gessel Apr. 11, 1944'2,363,531 Johnson Nov. 28, 1944 2,473,642 Found June 21, 1949 2,622,221Beese Dec. 16, 1952

